In warp knitting operations it is of critical importance that the feed yarn be delivered from a yarn beam at a prescribed rate. Otherwise, the yarn may be overslacked or overtensioned, causing defects and yarn breakage. Therefore, the rotational speed of the yarn beam must be carefully controlled to insure an appropriate rate of yarn feed. Controlling yarn delivery is complicated by the nature of the delivery mechanism, i.e., a linear, overlapped yarn being unwound from a cylindrical beam. As yarn is taken off of the beam, the effective diameter of the beam is reduced and, as a result, less yarn is delivered per revolution of the beam. Thus, in order to maintain a constant rate of delivery, it is necessary to increase the rotational speed of the beam in accordance with the reduction in beam diameter.
Devices have been developed to compensate for the dynamically decreasing beam diameter as discussed above. One such compensation device in wide use is the stepless variable cone gear let-off. Let-offs of this type function as continuously variable gearboxes for adjusting the gear ratio between the yarn beam drive means and the yarn beam. Adjustment is accomplished by adjusting the speed of a spindle extending from the let-off. If no force acts on the spindle and it is allowed to spin freely, then the gear ratio remains constant and the beam speed is not altered. The gear ratio and, thus, the speed of the beam are increased if the speed of the spindle is increased by an external drive means. The gear ratio and, thus, the speed of the beam are decreased if the speed of the spindle is decreased by an external drive means or inhibitor. After the gear ratio has been adjusted by manipulation of the spindle as discussed above such that beam speed and beam diameter result in a prescribed yarn delivery rate, the spindle is again allowed to spin freely until another adjustment needs to be made. A more detailed discussion of the operation and construction of a stepless variable cone gear let-off is discussed below in the detailed description of the preferred embodiment.
In order to provide feedback between the yarn beam and the let-off, mechanical control and feedback means have been implemented. Such means typically include a measuring arm having a roller in contact with the yarn on the beam. As the beam turns, the roller turns at the rate of travel of the yarn surface which is the same as the rate of the yarn delivery. A mechanical linkage, typically comprising chains and pulleys, connects the roller to the aforementioned spindle. When the rotation rate of the roller exceeds that of the spindle, the spindle speed is increased. When the rotation rate of the roller is less than that of the spindle, the spindle speed is inhibited.
Mechanical control and feedback means as described above suffer from several significant drawbacks. Because the entire feedback system is mechanical, it is prone to wander from its original settings, requiring periodic checks and readjustments. Further, if the roller is fomed of metal or some other hard and durable material, it tends to slip on the yarn, providing inaccurate feedback. If the roller is made from rubber or similar material, it tends to wear down, resulting in a smaller diameter roller and, again, inaccurate feedback. Moreover, the mechanical linkage typically includes a chain and a geared pulley affixed to the spindle. In the knitting environment, there exists a tendency for fiber, dust, and yarn remnants to accumulate about the chain and gear pulley. Thus, the mechanical linkage must be periodically cleaned. Such mechanical feedback means do not lend themselves to convenient modification of settings.
Thus, there exists a need for a more reliable means for controlling stepless variable cone gear let-offs which overcomes the deficiencies in accuracy, maintainability, and variability of the mechanical feedback means of the prior art.